An arrangement of this type is known through for example U.S. Pat. No. 5,532,575 and the article “A Multilevel Voltage-Source inverter with Separate DC Sources for Static Var Generation”, 1995 IEEE, pages 2541-2548. A Voltage Source Converter having phase legs formed by a so-called chain-link of H-bridge cells each having at least one energy storing capacitor is there used in an arrangement for exchanging power with a three-phase electric power network. The advantage of such a use is that already at a comparatively low number of such switching cells connected in series a comparatively high number of different levels of said voltage pulse delivered by the converter may be obtained, so that a said voltage with fundamental frequency having a shape being very close to a sinusoidal voltage may be obtained already without any smoothing filters. Furthermore, this may be obtained already by means of substantially lower switching frequencies than used in two or three level Voltage Source Converters. Furthermore, this makes it possible to obtain substantially lower losses and also reduces problems of filtering and harmonic currents and radio interferences, so that equipment therefor may be less costly. This altogether results in both a better performance of the arrangement and saving of costs with respect to such arrangements having converters with no energy storing capacitors built in into the switching cells.
It is of importance for reliable and efficient operation of such an arrangement that the energy stored in the capacitor of each switching cell of the converter is kept constant for keeping the direct voltage of each said cell constant. This implies that the converter in this known arrangement having wye-connected phase legs can only generate reactive power, i.e. the current component is a positive-sequence current and is in quadrature to the voltage at the respective phase leg terminal of the converter. This is due to the fact that when a negative-sequence current is generated by a wye-connected converter and a positive-sequence voltage is assumed, then the energy in said capacitors will increase/decrease between phases resulting in direct voltage variations across the capacitors.
However, it may in some situations be very important to be able to create a negative-sequence current in order to move power from one of the phases of the power network to the other for obtaining balance of the power flow in an instable network or otherwise reducing disturbances in the network.
It is of course an ongoing attempt to improve arrangements of the type defined in the introduction.